Despite the best efforts of staff to "dampen down" the prince's eagerness to publicise his views on sensitive subjects, it was difficult to argue he was "not political", said Mark Bolland, his former deputy private secretary.
The prince's self-appointed "campaigning role" was "constitutionally controversial", claimed Mr Bolland and had not "so far as I am aware, been endorsed either by the Queen or by Parliament." It was "regarded with concern by politicians".
Mr Bolland, 39, who worked for the prince for five years until 2002, is the newspaper's key witness. His testimony became public after the prince's lawyers yesterday dropped their attempt to have parts of it heard in secret.
The prince claims the publication of extracts from the journal - which he subtitled "The Great Chinese Takeaway" and which he claims were improperly copied by a "disloyal" former secretary at his private office - is a breach of confidence and copyright.
The Mail on Sunday claims he has "gone political", and therefore his views are of public interest. Mr Bolland told Mr Justice Blackburne the prince told him his "very definite aim" was to "influence opinion", through meetings with ministers and other people of influence and through speeches and using the media.
"He says that in making these views heard, he is always careful to avoid issues which are politically contentious," said Mr Bolland.
"Despite our best efforts, he did not always avoid politically contentious issues if he felt strongly about particular issues or Government policies."
Mr Bolland, who claims he had "direct access" to the prince and was Camilla Parker Bowles's "de facto private secretary", cites the prince's "vociferous" campaign against GM food as an example. Different private secretaries took different approaches to "this thorny issue", he said.
Richard Aylard, who preceded Sir Stephen Lamport and Sir Michael Peat, promoted the prince as "a wise man, a thinker and a changer of views". Sir Stephen, with Mr Bolland as his deputy, "tried to dampen down" the prince's behaviour in making public his thoughts, which they believed conflicted with the "monarch's constitutional role".
The Mail on Sunday claims the prince's journals were not confidential and were widely circulated among staff and friends. It also claimed the prince made clear his views on China by boycotting a banquet hosted by President Jiang Zemin during his state visit to London in 1999.
Mr Bolland claimed: "The prince chose not to attend the return state banquet at the Chinese embassy but to attend instead a private dinner at his home with Camilla Parker Bowles and friends."
He said the prince did this as "a deliberate snub".
"I was given a direct and personal instruction by the prince to draw to the media's attention his boycotting of the banquet". This he did by briefing The Daily Telegraph.
In his statement on behalf of the prince, Sir Michael dismissed Mr Bolland's claims. The prince, he said, "avoids making public statement on matters which are the subject of disagreement between political parties".
"Speeches and articles are cleared beforehand with the relevant Government department. He does, from time to time, express views privately to Government ministers. He has followed the practice that such views can be expressed by the monarch and by privy councillors.
"The Prince of Wales has not 'bombarded' ministers with his views but has written to them from time to time on issues which he believes to be important."
"I am informed by him that he gave no instruction to draw the media's attention to his failure to attend that banquet or to publish material critical of the Chinese government."
He said only 14 copies of the Hong Kong Journal were sent to 21 recipients, counting husbands and wives as one, and it was clearly a confidential and private document.
The prince claims the case should not go to full trial as the newspaper has no defence.